Eosinophils are multifunctional cells involved in parasitic, bacterial, and fungal infections, immunoreglation, allergic reactions, and asthma. They usually have a bilobed nucleus and contain distinctive granules.

• Crystalloid Granules (orange) - numerous ellipsoid granules with a linear (electron-dense) cyrstalline core.
• Cytotoxic cationic proteins (responsible for staining with eosin), ribonuclease and antiviral activity, cytokines, and chemokines.
• Eosinophil peroxidase needed to convert hydrogen peroxide (H₂O₂) into highly toxic hypobromous acids (HOBr; similar to bleach).
• Release their contents by classical exocytosis, piecemeal degranulation, or cytolysis.
• Small Granules (black) - few round (electron-dense) granules that are secretory lysosomes.
• Lysosomal degradative enzymes
• Additional cell-specific proteins (e.g., Charcot-Leyden crystal protein)

Although an eosinophil contains numerous granules, it contains relatively few organelles.

• Plasma Membrane (dark green) - extends small processes (pseudopodia) involved in amoeboid movement.
• Membrane-bound NADPH oxidase needed to produce toxic oxygen radicals and hydrogen peroxide (H₂O₂).
• Nucleus (blue) / Nuclear Envelope (purple) - unlike this cell it is usually bilobed.
• Golgi Apparatus - poorly developed (not visible).
• Mitochondria (red) - small, few in number.
• Endoplasmic Reticulum (cyan) - scattered fragments.
• Vesiculotubular System - small round or larger tubular vesicles involved in piecemeal degranulation.
• Glycogen (dark green) - numerous clusters of small, dark particles.
• Cytoplasm (green)

Eosinophils can use anaerobic glycolysis of the abundant glycogen as an energy source. Useful in the low oxygen environment of inflamed (or damaged) tissue.

Granule Secretion

Upon activation eosinophils release the contents of specific granules by three distinct mechanisms.

• Classical Exocytosis (anaphylactic degranulation) - the entire contents are released by fusion of granules with the plasma membrane.
• Often seen in interactions with large, non-phagocytosable parasites.
• Piecemeal Degranulation - small (round or curved) vesicles are used to transport "bits and pieces" of granules to the plasma membrane.
  • Allows the slow release of granule contents.
  • Progressive emptying leaves empty granules otherwise intact.
  • Number of granules involved varies.
  • Most common type of secretion.
• Cytolysis - the contents of granules are released when the cell ruptures or undergoes necrosis.
• Major mechanism in human airway tissues.

Extracellular Killing

Eosinophils damage and kill pathogens that are too large for ingestion by phagocytosis. Upon activation, the contents of crystalloid granules are most often released by piecemeal degranulation.

• Cytotoxic cationic proteins.
• Several are crucial in the killing of parasites.
• Generation of extracellular cytotoxic oxidants on the cell surface (i.e., respiratory burst).
• NADPH oxidase (in the plasma membrane) produces toxic oxygen radicals and hydrogen peroxide (H₂O₂).
• Eosinophil peroxidase (delivered to the plasma membrane from specific granules) converts hydrogen peroxide (H₂O₂) into highly toxic hyphobromous acid (HOBr; similar to bleach).

These highly toxic molecules damage pathogens as well as the host tissue.